The vast majority of sail systems employed in the exploding field of boardsailing are characterized by two booms, one on each side of the sail. Such booms are often termed "a wishbone boom". The contruction and mounting of the wishbone boom and associated sail are such that the lee (downwind) part of the wishbone boom presses into, creases, and distorts the shape of the sail. This creasing and distortion causes air to flow such that it has a crosswise (up and down) component. The path of the air flowing along the sail is thereby lengthened, which creates extra drag without at the same time creating extra lift. Sail efficiency is therefore reduced.
Referring particularly to the leeside (downwind side) of the sail, it is emphasized that a very large portion of total sail force is created by the lower pressure on such leeside, thus the lee is extremely important and anything that disturbs airflow along such side creates a disproportionately large loss of sail efficiency. Thus, the distortion caused by the lee component of the wishbone boom, as it contacts the sail, has a distinct adverse affect on sail efficiency.
It is further emphasized that the boom has a wake, namely a turbulent area behind its leeside. Such turbulent area extends downstream for several diameters of the boom. When a boom component is placed close to the sail on the leeside thereof, as is the situation relative to the lee component of a wishbone boom, there is turbulence and consequent flow separation immediately behind the boom. Such separation combines with air separation from the leeside of the sail caused by the distorted shape thereof. The separation is greater when the sail is tilted, as is conventionally done during boardsailing, in that the boom component (on the leeside) presents a maximum surface area across the airflow.
It is also pointed out that the boom itself creates drag, and that anything that reduces the overall size of the boom reduces such drag.